In electrostatography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) is formed on an insulative surface by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise radiation-induced discharge of a uniform potential previously formed on a surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of a pattern of electrostatic potential on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer (if desired, the latent image can be transferred to another surface before development). The resultant toner image can then be fixed in place on the surface by application of heat and/or pressure or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred by known means to another surface, to which it then can be similarly fixed.
In many electrostatographic imaging processes, the surface to which the toner image is intended to be ultimately transferred and fixed is the surface of a sheet of plain paper or, when it is desired to view the image by transmitted light (e.g., by projection in an overhead projector), the surface of a transparent film sheet element.
Transparent electrostatographic-toner-image-receiving elements are generally well known in the art of electrostatography. They often comprise a transparent substrate sheet having on one or both sides thereof a transparent image-receiving polymeric binder layer. See, for example, U.S. Pat. Nos. 4,873,135; 4,869,955; 4,526,847; 4,481,252; 4,480,003; and 4,415,626, the disclosures of which are hereby incorporated herein by reference.
One recurring problem with polymeric-binder-coated film sheets involves feeding the sheets through electrophotographic copiers. While most copiers contain apparatus that will fairly reliably feed plain paper through the machine, such apparatus often fails to feed polymeric-binder-coated film sheets through the machine with as high a degree of reliability. Failures often occur in the form of misfeeds, i.e., failure of the feeding apparatus to successfully remove one sheet from a stack of such sheets and properly direct it through the sheet-transport path in the machine, often resulting in a jam in the machine.
The problem is recognized in the art, and attempts have been made to overcome it. Such attempts have often involved adding discrete particles of various materials to the image-receiving polymeric binder layer, such that some of the particles protrude from the outer surface of the layer, in order to lessen the degree of contact between sheets in a stack and thereby allow them to move over each other more easily, and in order to provide a rougher surface to the sheets to increase the amount of friction between the sheets and sheet-feeding apparatus to thereby improve the ability of the apparatus to transport the sheets properly. See, for example, all of the U.S. Patents identified above.
While such added particles do generally reduce the frequency of misfeeds, in many cases the reduction is not enough to reach desired levels of reliability. Also, levels of reliability can vary with the particular type of feeding apparatus in particular types of copiers, so that even though a particular type of receiving element may feed very reliably in one particular type of copier, it may feed much less reliably in another particular type of machine.
Thus, there is a continuing need to provide improved transparent image-receiving elements that will exhibit higher levels of feeding reliability in electrostatographic imaging apparatus and, preferably, that will exhibit such high levels of feeding reliability in various different particular types of imaging apparatus.